A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid via a point of common coupling. Microgrids are part of the structure for so called distributed generation (DG) aiming at producing electrical power locally from many small energy sources which may be called distributed generators (DG:s) or micro sources.
In a microgrid, system stability is improved with application of energy storage for continuous real and reactive power injection that works as a stabilizer for the microgrid. The main control philosophy for such stabilizer is real and reactive power injection based on local frequency and voltage deviation, respectively. In most scenarios, a larger storage/stabilizer is economical. However, in a microgrid, depending on growth, expansion and with higher penetration of DGs, it may be required to add a new storage/stabilizer in an existing microgrid and that leads to scenarios with multiple stabilizers in the same microgrid. Moreover, a planned multiple stabilizer scenario can also be beneficial for a microgrid with critical loads and frequency dependencies. This scenario is also realistic considering the DG participation in system damping.
In an alternating current (AC) system, the frequency is the same everywhere in steady state while voltage may differ depending on the power flow. However, in a microgrid with a continuous change in DG output, load switching and low inertia, there is continuous frequency and voltage fluctuation to a small scale. And the deviations are larger during large transients (like DG fault etc.).
In an AC system, frequency and voltage stability relates to minimum oscillations and overshoot with ability to come back to initial value (or any other steady state value within acceptable deviation) after a disturbance. Thus, microgrid stability could be improved with a more tight regulation of voltage and frequency. One way of doing that would be more sensitive stabilizers e.g. higher feedback gains. Unfortunately that has negative consequences in terms of system damping limited by the grid components time constants and controller bandwidth. On the other hand, the stabilizer acting against the large disturbances must be very fast and controller action must take place promptly and accurately to eliminate the control error.